1. Field of the Invention
The present invention relates to a method for barrel plating by sequentially dipping a variety of small pieces of different sizes and kinds, such as nuts and bolts stored in a plurality of barrels, into various treatment tanks.
2. Description of the Related Art
Barrel plating apparatuses have been widely used for zinc-plating or the like of small pieces, such as nuts, bolts, and the like. An example of such barrel plating apparatuses is disclosed in U.S. Pat. No. 4,769,117. This disclosed barrel plating apparatus has a plurality of barrels for containing small pieces, and performs plating by sequentially dipping the barrels into several treatment tanks, such as a rinse tank, a degreasing tank, an acid cleaning tank, a neutralizing tank, a plating tank, another rinse tank, etc., while rotating the barrels. The pieces contained in the barrels are supplied with plating current from electrodes provided in the barrels, while the barrels are dipped into the plating tank.
In order to perform good plating, it is necessary optimally to control the plating current. For small pieces, such as nuts, bolts and the like, empirically obtained plating current values for representative sizes are stored in a table. Based on data in the table, an operator sets a value of the plating current with appropriate correction based on the operator's own empirical knowledge. However, this conventional method has a problem with variations in the setting of plating current by different operators, resulting in inconsistent plating quality.
When plating nuts and bolts used for motor vehicles, it often becomes necessary to plate many types of items in a continuous operation by placing each type of item into separate barrels, rather than serially plating a single type of nut or bolt contained in many barrels. This prior art manual plating current setting method requires an operator to set a plating current for each of various items that continually arrive at an input station. Thus, the prior art method is likely to cause human error when setting the plating current.
Furthermore, in a typical conventional barrel plating apparatus, the power sources for supplying plating current to barrels are not in one-to-one correspondence with the barrels. That is, one power source supplies plating current to a plurality of barrels. Therefore, if neighboring barrels contain different types of items that considerably differ in the required plating current per barrel, the plating film thickness of pieces can become excessively thick in one barrel and too thin in another barrel. Consequently, it becomes difficult to employ a plating method in which each barrel contains a different type of items.